The invention relates to optical means for remotely measuring pressure and, particularly, for invasive, or direct, measurement of arterial blood pressure.
In medicine, invasive measurement of arterial blood pressure is necessary in the management of critically ill patients or those undergoing complex surgical procedures. Two methods are currently available for direct blood pressure measurements. The first, and most widely used, involves insertion of a fluid-filled catheter into an artery so that the arterial lumen is hydraulically coupled to an external pressure sensing device. Highly accurate arterial pressure-pulse measurements are difficult, if not impossible, however, because the natural frequency of the hydraulic coupling coincides with frequencies of physiological interest. Moreover, the frequency response is variable, depending on the length of the catheter tubing and other unpredictable factors, such as the presence of small bubbles, leaks, or overly compliant plastic tubing used as connectors. These latter factors have impeded attempts to electronically compensate for the undesirable characteristics of the hydraulic coupling.
The other method of direct arterial blood pressure measurement involves the use of miniature solid state or strain-gauge transducers mounted on the tip of a catheter. Although catheter-tip manometers introduce little or no distortion into the pressure signal, a number of practical problems restrict their routine clinical use. The transducers are expensive, and their fragility limits the number of uses for a single catheter. They exhibit DC electrical drift, requiring the use of a fluid-filled lumen or separate catheter to obtain absolute values of arterial pressure. Also, there have been reported instances of mechanical failure of the catheter tip, introducing additional clinical hazards.
In the area of industrial process control, monitoring reactor-vessel pressure is critical for safe and automated operation of nuclear power plants. Pressurized water and boiling water reactors operate at pressures ranging from 1000-1500 psi and temperatures ranging from 250.degree.-350.degree. C. Such conditions, together with the corrosive effects of water and high radiation levels, limit the choice of sensors available for monitoring pressure. Mechanical pressure transducers, such as bellows and diaphragms, are frequently used. However, the transducers are typically external to the reactor vessel, and require that pressure signals be transmitted through fluid conduits. Elimination of such instrument piping is highly desirable where toxic or corrosive fluids are involved, or where even minor leaks lead to severe disruptions in plant operation. In addition, tubing interposed between the point of measurement and the transducers adversely affects the system's frequency response.
Strain-gauge pressure transducers are highly accurate and can be used in hostile environments. However, there are drawbacks to their use. If pressure measurements must be precise, and there are wide and sudden changes in ambient temperature, thermal protection is necessary. High-pressure spikes such as those caused by rapid opening or closing of valves can damage the transducers. Finally, signal transmission from a strain-gauge transducer to point of readout is by electrical wiring. While this eliminates the response time lag encountered whenever a fluid signal-transmission medium is used, electrical wiring is subject to corrosion heat damage, and breakage.
Many of the above-mentioned difficulties with current pressure-sensing technology can be overcome by using remote, in situ mechanical transducers coupled to a detector by optical waveguides, or fiber optics. Fiber optics are durable, corrosion-resistant, heat-resistant, and are available in very small diameters, which makes them amenable for use with miniaturized transducers.
Brogardh, in U.S. Pat. No. 4,270,050, dated May 25, 1981, discloses a remote pressure-sensing device which employs a transducer connected to a detector by a fiber optic. Pressure is sensed by measuring stress-induced changes in the absorption spectrum of a material placed in the path of an illumination beam at the site of the transducer. The transducer includes the stress-sensitive material and a means for converting pressure into a mechanical stress directed to the stress-sensitive material.
A problem with materials that have stress-dependent absorption spectra is that the spectral changes are also temperature dependent. Thus, for reasonable accuracy over appreciable temperature ranges, temperature stabilization is required. Another problem involves the need for converting pressure to stress on the sensitive material. The primary transducer for carrying out this conversion can impair the system's response time, and can increase the difficulty of miniaturization.
Ho, in U.S. Pat. No. 4,158,310, dated June 19, 1979, discloses a fiber optical pressure sensor which requires a cable of fibers and a deformable diaphragm having a reflective surface. The cable is divided at one end into two bundles, one of which is irradiated by a light source, and the other which directs reflected light to a detector. The irradiated fibers are distributed randomly among the fibers of the undivided part of the cable, which in turn is directed to the reflective surface of the deformable diaphragm. The other side of the diaphragm is in contact with the pressurized medium. The curvature of the diaphragm increases in response to increases in pressure, so that less light is reflected onto the fibers leading to the detector. Thus, the intensity of light collected by the detector varies inversely with the ambient pressure.
The use of a fiber cable and diaphragm makes miniaturization difficult, if not impossible. Cables also substantially increase the cost of the sensor, especially in remote sensing applications.
The foregoing illustrates the limitations of the current pressure-sensing technology. An alternative to available pressure sensing methods which overcame some of these limitations would be highly advantageous for remote pressure sensing applications, particularly in situ measurement of arterial blood pressure.